The present invention relates generally to techniques and systems for frequency acquisition and tracking and, more particularly, to frequency acquisition and tracking in direct-sequence spread-spectrum (DSSS) code division multiple access (CDMA) systems.
Wireless communications is expanding at a phenomenal rate, as more radio spectrum becomes available for commercial use and as cellular phones become commonplace. In addition, there is currently an evolution from analog communications to digital communications. In digital communications, speech is represented by a series of bits, which are modulated and transmitted from a base station to a phone. The phone demodulates the received waveform to recover the bits, which are then converted back into speech. There is also a growing demand for data services, such as e-mail and Internet access, which require digital communications.
There are many types of digital communications systems. Traditionally, frequency-division-multiple-access (FDMA) has been used to divide the spectrum up into a plurality of radio channels corresponding to different carrier frequencies. These carriers may be further divided into time slots, referred to as time-division-multiple-access (TDMA), as is done in the D-AMPS, PDC, and GSM digital cellular systems.
If the radio channel is wide enough, multiple users can occupy the same channel using spread spectrum (SS) techniques and code-division-multiple-access (CDMA). IS-95 and J-STD-008 are examples of CDMA standards. With direct sequence spread spectrum (DS-SS), information symbols are multiplied by sequences of symbols referred to as chips. This multiplication spreads the information symbols in the frequency band. At the receiver, correlations to the chip sequences are used to recover the information symbols. Spreading allows the system to operate at a low chip signal-to-noise ratio (SNR). If thermal noise is not too great, then noise from other users is tolerable. Thus, multiple user signals can occupy the same bandwidth at the same time, giving rise to CDMA.
Digital communication receivers typically include a radio processor and a baseband processor. The radio processor filters, amplifies, and mixes the radio signal down to baseband. At some point the signal is sampled and quantized, ultimately providing a sequence of baseband received samples. Since the original radio signal has in-phase (I) and quadrature (Q) components, the baseband samples typically have I and Q components, giving rise to complex, baseband samples. Baseband signal processing is then used detect the bits that were transmitted.
In the mixing down operations, mixing is based on a reference oscillator and knowledge of the transmit carrier frequency. Due to manufacturing and temperature variability, the output of the reference oscillator is not exactly at the desired, fixed frequency. As a result, the radio signal is not mixed exactly to the desired baseband frequency (typically 0 Hz). This gives rise to frequency error which degrades performance.
In narrowband systems, there are a variety of techniques for acquiring and tracking frequency error. If these narrowband techniques were applied to chip values in a CDMA system, poor performance would result because of the extremely low chip SNR.
Frequency acquisition and tracking techniques designed for direct sequence spread spectrum receivers have been developed. One approach is given by an article authored by Mauss et al., entitled "Carrier frequency recovery for a fully digital direct-sequence spread-spectrum receiver: a comparison" and found in VTC '93, Secaucus, N.J. In this article baseband samples are first despread, using knowledge of the spreading sequences, giving rise to a sequence of despread values. A differential detector is then applied to the sequence of despread values to form a sequence of detector outputs. The detector outputs are complex numbers in rectangular coordinates, which can be viewed as having an amplitude and a phase angle. The sequence of complex detector outputs are converted into a sequence of amplitude and phase angle values. The amplitude values are modified by some arbitrary function f. The modified amplitude and original phase angle are next converted back into rectangular coordinates. The modified detector outputs are summed over time and the phase angle of the sum is taken and scaled to give an estimate of the frequency error. If the despread values correspond to known symbols, then the function f replaces the amplitude with known differential symbol values. Otherwise, the amplitudes are not replaced. The frequency error can be estimated periodically and filtered to obtain a smoothed estimate, which can be used to adjust the reference oscillator.
For initial frequency acquisition, this approach is limited by the amount of coherent integration provided by despreading prior to differential detection. Only one symbol period of coherent integration is employed, followed by differential detection, which amplifies the noise.
Another approach is given in an article authored by U. Fawer, entitled "A coherent spread-spectrum diversity-receiver with AFC for multipath fading channels", found in IEEE Trans. Commun., vol. 42, pp. 1300-1311, February/March/April 1994. According to this article, frequency error estimation is performed after channel (phase) estimation and Rake combining. However, frequency error estimation is better performed before channel estimation because frequency errors will degrade channel estimation. Thus, there continues to be a need to accurately estimate and track frequency error in direct-sequence spread-spectrum receivers.